1. Field of the Invention
This invention relates to an improved composition and method for providing a thermally releasable barrier coating for surfaces not intended to be painted but on which paint.sup.1 nevertheless accumulates in practice because of the proximity of the surfaces not intended to be painted to other surfaces that are intended to be painted. The invention is more particularly applicable for coating the floor grates in paint spray booths or cabinets, such as are extensively used in manufacturing automobiles and many other objects that are decorated by painting.
2. Discussion of Related Art
Numerous barrier coatings have been used in the prior art. Most have been removed when necessary by solvents, use of high pressure and/or high temperature water, mechanical means, or the like, all of which to greater or lesser degrees are labor intensive, require substantial volumes of at least moderately expensive materials, or both. More recently, thermally removable barrier coatings have become known in the art.
Most of those so far known utilize a combination of carbonate and/or acid carbonate salts FNT .sup.1 Throughout this description, unless the context requires to the contrary, as where a specific material is named, the term "paint" is to be understood as encompassing all similar terms such as enamel, lacquer, shellac, varnish, and the like. with an acidic material that is prevented from coming into contact with the carbonate even when mixed with it at normal ambient temperatures, but that is brought into contact by some mechanical and/or chemical means when the temperature of the solid containing both components is raised, most often by contact with hot water and/or steam. The resulting reaction between acid and carbonate generates carbon dioxide gas that causes the coating to rupture and in principle to fall off the coated surface, or at least to become removable by light mechanical action.
These thermally removable barrier coatings have diminished the labor and/or material costs compared with those of previous methods of removing protective barrier coatings, but in practice have been found to have certain difficulties: Complete removal is sometimes not reliably achieved; the paint accumulated on the barrier coating surface often becomes detached during the removal process, so that it can redeposit on the surface intended to be protected from being coated with it; pieces of the coating after release can all too readily become trapped in the structure having the surface to be protected from paint, particularly when this structure has relatively narrow spacings between solid elements, as is true in the grates conventionally used in paint spray booths; and the internal barrier materials that prevent contact between carbonate ions and acid materials at ambient temperatures can become unstable or otherwise suffer faults that cause premature removal and/or unreliable or incomplete removal when desired.